Photopolymerizable compositions have for several years been supplied commercially in layer form sandwiched between a support film and a cover sheet for use as a photoresist in the manner described in U.S. Pat. No. 3,469,982. This photoresist element has been supplied in roll form to users of the photoresist such as printed circuit manufacturers. The cover sheet prevents the photopolymerizable layer from sticking to the backside of the support film under the interlayer pressure normally present in the roll, thus enabling the element to be unrolled for photoresist use.
Cold flow of the layer has frequently been a problem, however. By cold flow is meant flow of the layer at ambient temperature conditions. The photopolymerizable layer has some tendency, depending on composition and layer thickness, to flow out the ends of the roll during storage. In extreme cases, adjacent layers come together at the roll ends and bond together to prevent unrolling of the layers.
U.S. Pat. No. 3,867,153 discloses photopolymerizing the ends of the photopolymerizable layers in the roll, which prevents cold flow of the layers out from the end of the roll. While this technique is successful, it necessarily involves a further manipulative step and thus higher cost of the product.
It is also desirable to omit the cover sheet from the photoresist element since this too would be a saving in manufacturing cost. Without the cover sheet, however, the photopolymerizable layer can flow into intimate contact with the backside of the support film, thereby tending to adhere to it and prevent unrolling of the roll.
It is, of course, recognized in the prior art that simply by reducing level of monomer/plasticizer or by using a higher molecular weight binder, a photopolymerizable layer having reduced cold flow can be obtained. However, this approach results in slower photopolymerization and washout (development) rates. Thus, there remains a definite need for a really satisfactory solution to the problem of cold flow.